The present invention relates to a new cemented carbide grade with excellent properties especially for tools in the wood industry. More particularly, the invention relates to a cemented carbide in which submicron WC has been distributed in a monophase binder phase based on Ni, Co and Cr.
Reconstituted wood products, such as medium density fiberboard and chipboard, are the main raw materials in the furniture industry. They are also used to some extent in the housing industry.
These products are machined with a variety of tool materials, from high speed steels to cemented carbide to polycrystalline diamond. A leading role has been played and is still being played by tools made with cemented carbides.
The composition of the cemented carbide grades used for wood working tools consists generally of tungsten carbide (WC), as the hard component, and cobalt (Co) as a binder to hold together the WC crystals. Sometimes small amounts of other carbides, like titanium carbide, tantalum carbide, etc., are added.
To satisfy the different demands on hardness and toughness, the amount of Co and/or the grain size of the WC are varied. Higher Co-content and/or larger grain size decrease hardness and increase toughness.
Mechanical wear, especially abrasion, has been thought to be the primary mechanism of tool wear when machining reconstituted wood products. Recent work, however, has proven that chemical mechanisms such as corrosion and oxidation play a significant role in tool degradation. The same is valid also for tools for machining of printed circuit boards and similar composite materials.
As these wood products are machined, the tool temperature can increase dramatically. As the temperature increases, the wood products go through thermal breakdown resulting in the introduction of numerous chemicals into the cutting environment. In all, up to 213 different compounds have been identified upon the destructive distillation of wood. The machining of medium density fiberboard and particle board produces even more decomposition products. These products not only have the wood fibers, but also a binder such as urea, formaldehyde, wax and glue fillers and extenders, and possibly chemicals added as flame retardants.
The decomposition products formed are highly corrosive and attack the Co-binder that holds the WC grains together. When this occurs, the WC grains are removed by mechanical action and the tool cutting edge loses its sharpness and its cutting capability.
The high temperature achieved when machining wood products contributes also to the degradation of the binder by oxidation of the Co in air.
What has been said above is also valid when cutting green lumber or dried wood. Both products contain moisture and organic acids capable of dissolving the Co-binder and therefore weakening the bond holding the carbide particles in place until the exposed particles are removed mechanically.